Many processes require measurement and control of the flow rate of small amounts of a liquid. Such processes include those employed in the life sciences, chemical analysis (such as liquid chromatography) biotechnology, chemical synthesis and nanotechnology.
Some liquid flow sensors determine a flow rate by utilizing the thermal behavior of a liquid passing through a section of tubing. For example, some nano-flow sensors measure liquid velocity by determining heat transfer rate. Such a sensor typically observes a flow-rate related variation of a fluid's temperature along the tube.
One type of commercially available low-flow thermal flow-rate sensor uses two active elements, each of which acts as both a heater and a temperature sensor. The temperature difference between the two elements provides a measure of flow rate. FIG. 1 is a block diagram of such a flow sensor 100. The sensor 100 includes a flow tube 110, and two active elements 120, 130 disposed in contact with the tube, upstream and downstream relative to each other. For low flow-rate chromatography applications, the tube 100 is typically formed of stainless steel, a polymer or fused silica. Depending on the flow-rate range of interest, the internal diameter is in a range of approximately 25 μm to 250 μm, and the internal volume is in a range of approximately 20 nl to 2 μl.
The active elements 120, 130 are wire coils, wound around the tube 110. The elements 120, 130 are both heat sources and temperature sensors. The elements 120, 130 introduce heat to a fluid in the tube 110 when a current is passed through the coils; their temperature is measured by monitoring the resistance of the coils, which changes with a change in temperature.
The flow sensor 100 is typically operated in a constant-power mode, for flow rates below approximately 100 μl/min, and in a constant-temperature mode, for higher flow rates. In the constant power mode, equal power is supplied to both elements 120, 130, and their temperature readings are used to determine the flow-rate.
In the constant-temperature mode, the upstream element 120 acts as a heater and the downstream element 130 need only act as a temperature sensor. Sufficient power is delivered to the upstream heater 120 to maintain a constant temperature difference, ΔT, between the upstream element 120 and the downstream sensor 130. The supplied power provides a measure of the flow rate.
The flow sensor 100 utilizes an electronic circuit based on a Wheatstone bridge configuration. The circuit converts the output signals, associated with ΔT or power, into an output voltage that has a linear relationship with the mass flow of liquid in the tube 110.
The temperature at each element 120, 130 depends on the ambient environment, electrical current, coil resistance, conduction of heat through the tubing, conduction of heat through the fluid in the tube 110, convection of fluid in the tube 110 (typically dominated by flow) and, probably to a lesser extent, heat transmission between the surface of the tube 110 and its surroundings.
The sensor 100 communicates with the ambient, notably, via heat transmission along the liquid and along the tube 110. Use of metallic tubing can lead to substantial conduction of heat along the tube 110. If the ambient temperature variations cause a difference in temperature between the elements 120, 130, a temperature difference is observed even when the flow rate is zero. This problem is exacerbated when the ambient temperature spatial distribution varies over time, causing a temporal variation of the temperature offset between the elements 120, 130. To mitigate this problem, some flow sensors include an aluminum container within which a flow tube 100 is disposed.
Flow sensors can be calibrated at different temperatures to help compensate for ambient-temperature effects. Available sensors also typically have an output that is different for different liquids, such as the different solvents used in some types of chromatography. A correction factor can be applied to compensate for this effect. A correction factor, however, may be unavailable for a particular liquid, or use of a mixture of different liquids can complicate this problem.